Britain is scheduled to leave the European Union at 11 p.m. London time on March 29. But Brexit, in the sense of the full extrication of the U.K. from the bloc’s economic orbit, looks more and more likely to take many years.

A transition period has already been established in principle with Brussels that would, so long as there is an overall divorce agreement, keep the U.K. in the EU’s customs union and its single market until December 2020. That combination should keep two-way trade flowing as freely as it is now.